This application is based upon and claims the benefit of Japanese Patent Applications No. 11-242096 filed on Aug. 27, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a raindrop sensor suitably used for a vehicle wiper automatic control device.
2. Description of the Related Art
A conventional raindrop sensor is shown in FIG. 4. The raindrop sensor is attached to an interior wall of a vehicle windshield W to optically detect presence of raindrops. In the raindrop sensor, a light-emitting element 2 and a light-receiving element 3 are disposed at both sides in a longitudinal direction of a prism 1. The prism 1 has a plano-convex lens 1a facing the light-emitting element 2, a plano-convex lens 1b facing the light-receiving element 3, and a prism body 1c provided between the lenses 1a and 1b. 
The plano-convex lens 1a changes light from the light-emitting element 2 to parallel rays that are to be incident on the prism body 1c . Light incident on the prism body 1c is reflected several times between an exterior wall of the windshield Wand the central portion upperwall of the prismbody 1c as indicated by arrows in FIG. 4, and then enters the plano-convex lens 1b. Light from the plano-convex lens 1b converges to be incident on the light-receiving element 3. Incidentally, in FIG. 4, reference numeral 4 denotes a transparent adhesive layer.
In the raindrop sensor having the structure as described above, a light-emitting axis of the light-emitting element 2 and a light-receiving axis of the light-receiving element 3 must be respectively inclined at approximately 45 degrees with respect to the exterior wall of the windshield W to provide the optical path of light as described above. The light-emitting element 2 and the light-receiving element 3 are therefore provided on a wiring board 5 disposed above the prism 1 so that the light-emitting axis and the light-receiving axis have inclination of approximately 45 degrees with respect to the wiring board 5. The prism body 1c has two inclined surfaces on which the convex lenses 1a and 1b are respectively disposed so that optical axes of the convex lenses 1a and 1b respectively correspond to the light-emitting axis and the light-receiving axis of the elements 2 and 3.
In this connection, each of the inclined surfaces of the prism body 1c must have an area sufficient for holding each of the convex lenses 1a and 1b. This may result in unnecessary increase in height or length of the prism 1. In addition, since the light-emitting element 2 and the light-receiving element 3 project downward from the wiring board 5 via leads, the wiring board 5 must be positioned at a much higher position than the prism 1.
Further, when the raindrop sensor has several light-emitting elements to widen a detection region to improve detection accuracy, the raindrop sensor must have several light-receiving elements, a number of which corresponds to that of the light-emitting elements.
The present invention has been made in view of the above problems. An object of the present invention is to provide a raindrop sensor constructed with a compact structure with a decreased number of parts and having a widened detection region.
According to the present invention, briefly, a raindrop sensor has first and second light-emitting elements and first a and second incoming radiation side plano-convex lens portions provided on a prismbody to face the first and second light-emitting elements. The raindrop sensor further has a light-receiving element and first and second outgoing radiation side plano-convex lens portions. The first and second outgoing radiation side plano-convex lens portions are provided by dividing one, plano-convex lens into two pieces so that each of the first and second outgoing radiation side plano-convex lens portions has a divided surface. The divided surface of the first outgoing radiation side plano-convex lens portion faces the divided surface of the second outgoing radiation side plano-convex lens portion.
Accordingly, when the raindrop sensor is attached to a windshield, light emitted by the first and second light-emitting elements is incident on a wide region of an exterior wall of the windshield after passing through the first and second incoming radiation side plano-convex lens portions, and is reflected by the exterior wall. Then, light enters the first and second outgoing radiation side plano-convex lens portions, and is refracted toward the divided surface by each of the first and second outgoing radiation side plano-convex lens portions. As a result, light from both the first and second outgoing radiation side plano-convex lens portions can enter the light-receiving element.
The number of the light-receiving elements can be decreased largely as compared to that of the light-emitting element(s). As a result, the number of parts for the raindrop sensor is reduced. Since the outgoing radiation side lens portions are provided by dividing the one plano-convex lens, the size of the prism body for holding the lens portions can be reduced in a vertical direction. As a result, a gap between the prism body and a wiring board holding the light-emitting and light-receiving elements can be reduced resulting in decreases in height and length of the raindrop sensor.